On Wed, Feb 22, 2012 at 10:33:51AM -0800, Tejun Heo wrote:

[..]

quoted

So for blkio controller we can convert it into fully nested hierarchy
at the expense of more complex code in CFQ. I think memory cgroup
controller provides both flat and hierarchical mode. Keeping it fully
hierarchical also increases the cost as we need to traverse lot more
pointers for simple things like nested stats. On a system having
both systemd and libvirt, every virtual machine is already 3-4 level
deep in cgroup hierarchy.

I don't think every controller should implement full nesting and
sharing the same hierarchy doesn't require it.  ie. if a controller
only wants to support flat hierarchy, just allow a single subgroup to
be active on any path between root and leaf.  We can add a flag or
helpers to support such mode of operation and controllers themselves
can treat all cgroups equally.

I am not sure I understand "allow a single subgroup to be active on any
path"

So if a hierarchy looks as follows.

				root
			        / | \
			      g1  g2 g3
			          |   |	
				  g4  g5

So you are saying that just either g2 or g4 to be active in path 2 and 
similiarly allow g3 or g5 to be active. IOW, if a task is in g5 and g3
is active group, and effectively task will be considered in g3? So in
above diagram if g1 and g4 and g3 are active groups, controller will
see them as.

				root
				/ | \
			       g1 g4 g3

Did I understand it right or you meant something else. But this is still
not flat and has 2 level of hierarchy. Tasks in root group and tasks in 
children group (g1, g2 and g3) are different levels hence controller needs
to implement hierarchy. For it to be truly flat, it needs to look like
this.
			     pivot_point
				/ | \  \
			       g1 g4 g3 root

Now the notion that only one group is active in each path from root to
leaf does not mean much.

Considering everything internally as flat, isn't it simpler. So cgroup 
tree still might look hierarchical but actually controller treats it
as.
				root
		              / / | \ \
			    g1 g2 g3 g4 g5

Well, above is not exactly flat as has 2 level of hierarchy. That's blkio
controller views cgroup hierarhcy as follows, currently.

				pivot_point
		              / / | \   \  \ 
			    g1 g2 g3 g4 g5 root

Thanks
Vivek